Towards Reaching the Theoretical Limit of Porosity in Solid State Metal Foams: Intraparticle Expansion as A Primary and Additive Means to Create Porosity

Abstract

porosity has typically been limited to � 40%. These relatively low porosity levels (compared to liquid state processes) constrain the applications of metal foams produced via solid state foaming. To extend the capabilities of solid state foaming, we have developed an additive means of creating porosity by intraparticle expansion. The current limitation of solid-state expansion by gas entrapment is dictated by voids formed between solid particles during consolidation. In this model, the initial gas pressure and foaming temperature determine the resulting porosity. However, if the expanding gas is not limited to just that which is trapped between particles, but is also located within particles, solid state foaming may assume a character more akin to expandable polymers which foam from the constituent pellets. This concept is a paradigm shift in terms of the solid state foaming process and the achievable levels of porosity. On the basis of this simple, powder feedstock expansion, there is universal application to powder metallurgy methods for foaming. In this work, we examine the microstructure and morphology of a Cu–Sb alloy that expands to porosities of close to 40% within individual particles, resulting in absolute porosity of 69% in sintered samples. The Cu–Sb alloy powder was formed by mechanically alloying Cu and Sb (Alfa Aesar, 99.9 and 99.5%, respectively) at cryogenic temperature(� 196°C) for 4h using a modified SPEX 8000M Mixer/Mill. The elemental powders were combined to achieve 5 at% Sb in Cu. The as-milled powders contained no appreciable porosity. Ball milling was used as a means to intimately mix the elements and refine and distribute preexisting oxides. Although oxygen exposure was controlled during milling and storage of powders, the manufacturersupplied precursors did contain appreciable oxygen content. The importance of this will be detailed later. The alloyed powder was annealed at 600°C for a period of 1h under 3% H2 (bal. Ar). During annealing, the powders underwent pore formation and expansion. When annealing was done in the absence of H2, it did not produce any expansion. Microscopic examination of the loose powders was carried out using an FEI Nova Nano Lab 600 dual beam microscope using scanning electron microscopy (SEM). Cross-sectional analysis was performed using a focused ion beam (FIB). The grain size and grain orientations were measured using focused ion beam